The present invention relates generally to an optical fiber gyroscope, and more particularly to an optical fiber gyroscope which is specifically adapted to the measurement of an angular velocity in a rotary optical system.
Referring firstly to FIG. 1, there is shown a schematic block diagram which depicts a fundamental construction of a typical optical fiber gyroscope, as found, for instance, in the "Technical Material Collection on Optical Fiber Sensors (Hikari-fiber Sensor Gijutu Shiryo-shu)", p 263, issued Aug. 19, 1983, Diichi International K.K., wherein a beam of light emitted from a light source 10 such as a laser beam source is shown directed through a beam splitter 12a, a polarizer 14 and a beam splitter 12b to a condenser lens 15b, and then by this lens it is guided into a single-mode optical fiber loop 16, passing therethrough and comming out into the beam splitter 12b, where it is then passed on a return route through the polarizer 14 and then reflected by the beam splitter 12a so as to be eventually inputted to a photodetector 18. This generally describes the case where a light beam is directed through the optical fiber loop 16 in a clockwise direction as viewed in FIG. 1, and when passing in a counter-clockwise direction, it is notable only with a difference in the route of propagation such that the light beam is directed into the condenser lens 15a after being reflected by the beam splitter 12b, then led to pass through the optical fiber loop 16, and out on a return route through the condenser lens 15b and through the beam splitter 12b.
When an optical beam as shown in FIG. 1 is turned in a given direction at an angular velocity .OMEGA. with respect to the inertial space, there would be observed a difference in the propagation times of light beams passing clockwise and counter-clockwise through the optical fiber loop 16, and as a consequence, there would occur a phase difference .DELTA..theta. which may be expressed by the following equation and which is known as the Sagnac effect, as appeared in the above noted literature, that is: ##EQU1## where: l is the length of the optical fiber;
a is the radius of the optical fiber; PA1 c is the velocity of light (in the vacuum state); and PA1 .lambda. is the wave length of an incident light.
In consequence, when detecting both light beams passing through these clockwise and counter-clockwise routes by way of the photodetector 18 after both having been interfered with each other, it is noted that an electric signal outputted from the photodetector 18 will be a function of the phase difference .DELTA..theta., thus obtaining the value of .DELTA..theta., and thus deriving a desired value of angular velocity .OMEGA..
In this connection, while it is known that the output from the photodetector 18, according to the basic construction as shown in FIG. 1, changes in proportion to cos .DELTA..theta., one problem with this construction as reviewed generally above is that there is not attainable in practice a sufficiently good sensitivity which is enough to detect a fine rotation of the optical system.
According to the prior art, while there have been proposed to date many measures such as of using the optical phase modulation, the frequency modulation, the photo-heterodyne detection, etc., they would inevitably turn out with the following drawbacks:
(a) In an optical system where there are involved certain elements on one, i.e., a clockwise route of propagation for a light beam which are different from those of the other, i.e., counter-clockwise, route, there is available no efficient means to eliminate a substantial fluctuation as encountered due to the different routes in the system.
(b) While it is feasible to modify the construction of the photodector in such a manner that its output is in proportional to sin .DELTA..theta., instead of cos .DELTA..theta., the problem of a non-linearity in the relationship between the angular velocity .OMEGA. and a value of sin .DELTA..theta. would not be solved.
(c) If in practice there occurs a substantial difference in the frequencies of light beams when passed in the optical fiber loop clockwise on the one hand and counter-clockwise on the other hand, there would occur a substantial fluctuation in the outputs from the photodetector owing to changes in the temperature of the optical fiber loop.
(d) When there is adapted the frequency modulation by way of an acousto-optic light modulator (a light modulator device taking advantage of the isotropic Bragg diffraction), as a beat frequency of the output from the modulator would coincide with a given sound frequency, it would inevitably turn out to be complex in electric processings as to the operation thereof.